What Causes Miscarriages?

By CNY Fertility Updated on — Medically Reviewed and Certified by Dr. Edward Ditkoff Dr. Edward Ditkoff
What Causes Miscarriages?

Whether you’ve experienced a miscarriage or are just starting to try to conceive, you may be wondering what causes miscarriages.  While we don’t always know with certainty why miscarriages happen and the majority of couples don’t pursue additional testing, doctors believe that between 50-70% of miscarriages are caused by random genetic problems, or chromosomal abnormalities, in the developing baby chromosome abnormalities.

Most Common Miscarriage Cause: CHROMOSOME ABNORMALITIES

Chromosomes are the microscopic structures inside of our cells that carry our genes. We each have 23 pairs of chromosomes, and we get one set from each of our parents. When the egg and sperm combine, sometimes the chromosomes don’t line up properly, there can be extra or missing chromosomes, or the chromosome structure gets flipped or out of order. Geneticists have a name for each of these chromosomal abnormalities:

Those with abnormal chromosome numbers:

  • Polyploidy: Extra chromosome sets
  • Aneuploidy: An extra or missing chromosome
    • Monosomy: one chromosome absent
    • Trisomy: one chromosome extra

Those with abnormal chromosome structure:

  • Deletion: Part of a chromosome missing
  • Duplication: Part of a chromosome present twice
  • Translocation: Two chromosomes join long arms or join parts
  • Inversion: Segment of chromosome inverted
  • Isochromosome: A chromosome with identical arms
  • Ring Chromosome: A chromosome that forms a ring due to deletions in telomeres, which cause ends to adhere

The most common aneuploidy is Down syndrome, which is caused by an extra copy of chromosome 21 (three chromosomes 21 instead of two).  But given that many babies survive and are born with genetic conditions such as Down syndrome and other trisomies, why is it that some chromosomal abnormalities lead to miscarriage and others don’t?

Why do some chromosomal abnormalities cause miscarriage and others don’t?

CNY Fertility Reproductive endocrinologist Dr. Edward Ditkoff explains, “Unfortunately, we don’t know the answer to that.  One common theory is that the mother’s immune system recognizes a problem in the developing fetus and ends the pregnancy. Another possible explanation is that the developing baby ultimately reaches a point in development where the specific genetic code ends the development of the fetus at a particular time. Certain genes might be missing that are necessary for development or extra copies of certain genes might cause the baby or placenta to grow improperly. This could explain why certain types of chromosomal abnormalities lead to miscarriage while others don’t. Cell division is a complex process and not completely understood.” 

But for most women, miscarriage happens once since most chromosomal issues are random in nature. In the rare case where multiple miscarriages occur, it can be that there are chromosomal abnormalities in the fetus caused by one or both parents (a balanced translocation) that prevent a successful pregnancy without affecting either parent. In this case, genetic counseling would be recommended.

MATERNAL AGE: Correlated to Aneuploidy and Miscarriage Risk

The corollary to chromosome abnormalities is maternal age. Aneuploidy is a natural phenomenon that is related to age. “As women age, the aneuploidy rate increases,” says Dr. Ditkoff. “In an IVF setting women who are 36 years of age may be expected to have up to 50% abnormal blastocyst embryos, while at age 40 years of age the aneuploidy rate may be as high as 90% in the blastocyst, and as high as 95% in 42-year-old women. Aneuploidy rates are definitely related to early miscarriage rates.”

One recommendation according to Dr. Ditkoff is for women of “advanced maternal age is to use preimplantation genetic testing with IVF to exclude aneuploidy. by excluding embryos with aneuploidy, we are able to improve live birth rates while reducing miscarriage rates. Large data support preimplantation genetic testing for women between ages 37-42 years of age since their live birth rates have been shown to increase as a result of preimplantation genetic testing.”

But there is also evidence to suggest there are ways to turn back the clock and improve egg quality.  Providing the correct nutrients to the ovaries/eggs, balancing hormones, and reducing stress and inflammation can all improve egg quality. Dr. Robert Kiltz suggests patients take a prenatal vitamin, COQ10, eat a high fat/low carb diet to prevent inflammation, avoid toxins, and try acupuncture, massage, and yoga as a way to improve circulation to the ovaries and uterus and reduce stress inflammation. “All of these can help to improve egg quality, and while changes won’t happen overnight, making these lifestyle shifts can yield some impressive results,” says Dr. Kiltz.  Kiltz advises patients to switch to a ketogenic diet focusing on bacon, eggs, butter, beef, and full-fat ice cream for 3-4 months prior to IVF if possible as eggs start their final stage of maturation three months prior to ovulation.


Certain pre-existing health conditions have been known to contribute to miscarriage— Polycystic Ovary Syndrome (PCOS), diabetes, lupus, thyroid problems, high blood pressure (in late pregnancy). 

PCO can contribute to infertility by influencing ovulatory function and has been shown to be related to early pregnancy loss due to a lack of hormonal support (progesterone – which is usually triggered by the LH Surge often not present in those with PCOS) required during early pregnancy. 

What Causes Miscarriage? Infections Do

Some viral and bacterial infections can increase your risk of miscarriage. Several infectious illnesses are well-documented risk factors for miscarriage, but other studies suggest some common vaginal infections can also increase your risk.

  • Bacterial Vaginosis:  Bacterial vaginosis (BV) is a common vaginal infection. While it can produce symptoms including a fish-like odor, itching, burning after intercourse, and a white(ish) discharge, many with BV are symptom-free.  Studies have linked bacterial vaginosis to both first and second-trimester miscarriage.
  • STDs:  Sexually transmitted diseases including chlamydia can increase your odds of developing pelvic inflammatory disease (PID). PID is an inflammatory condition that is known to put women at risk factor ectopic pregnancy and increase miscarriage, potentially by altering the immune response in early pregnancy. 
  • Foodborne illness: Foodborne illnesses like Listeria and Salmonella have been linked to miscarriage risk.  
  • Toxoplasmosis:  Toxoplasma gondii, which can be found in cat feces can cause toxoplasmosis, which can cause miscarriage or congenital problems in developing fetuses. 
  • Parvovirus B 19 aka Fifth Disease:  Fifth Disease can cause a serious condition in a fetus if a non-immune pregnant mother is exposed causing miscarriage in 5% of those cases. As a relatively common disease, most mothers are immune due to childhood exposure. 
  • Rubella: Rubella (German measles) can cause congenital birth defects and miscarriage in the first trimester but is uncommon due to vaccination.
  • Chorioamnionitis:  Chorioamnionitis is a bacterial infection that travels to the cervix from the vagina. The infection is treated by antibiotics, but it can still be life-threatening for a mother and her baby. Doctors recommend immediate delivery to prevent additional complications.


The immune system is a lesser understood cause of miscarriage, but it may contribute to implantation failure and recurrent pregnancy loss. The mother’s immune system recognizes the fetus as an invader and treats it as such, eliciting an immune response that ultimately results in fetal demise. 

Treatment can involve the use of immunological medications which may include antibiotics, Intralipids, Lovenox, Prednisone, Low-dose aspirin, and Plaquenil. Some more aggressive treatments may include IVIG, Humira or Paternal Leukocyte Immune Treatment (LIT), also known as the male immunization treatment. Such immunologic treatments are still considered experimental but may be effective under proper clinical use and available at CNY.

Low Dose Naltrexone (LDN), thought to be a “miracle drug” because of the way it reduces inflammation and cannabinoid oil (CBD) is an herbal remedy that may also help in reducing inflammation.


Miscarriage can be caused by an incompatibility of the mother’s blood and the blood of her unborn fetus.  This is commonly known as Rh factor incompatibility. This type of miscarriage occurs when the mother is Rh-negative, and the fetus is Rh-positive. When there is a difference in the Rh factor between a mother and child, the mother’s body recognizes the fetus as a foreign body and can cause a miscarriage. Early testing usually makes this preventable; rhogam injections may be given to the pregnant patient to prevent antibody production.


Some women have congenital uterine abnormality (from birth) while others develop abnormal growths later in life. 

Common congenital uterine abnormalities are: 

  • Uterine Septum: the uterus is divided by a band of tissue which varies from patient to patient. This condition develops in the growing fetus and is present at birth. The septum may interfere with fetal growth and may cause implantation failure/miscarriage during the first trimester.
  • Bicornate Uterus: Commonly referred to as a heart-shaped uterus.
  • Unicornuate Uterus:  The uterus is half-shaped as one side did not develop in the fetus and therefore smaller than normal. Having this malformation is associated with an increased rate risk of miscarriage and pre-term delivery. 
  • Didelphic Uterus: This is a “double” uterus in that the two developmental horns do not fuse and having two small uteruses. This condition is quite rare.

Uterine Pathology:

  • Fibroids:  Technically a Uterine Pathology, Approximately 30-50% of women have uterine fibroids. Fibroids usually develop in adulthood. Depending on the type of fibroid and its location within the uterus, it can lead to miscarriage.  Fibroids that lie in the cavity are more concerning, as well as those that are larger in diameter. In other words, the size and location of the fibroid determine it’s clinical significance. 


When a woman’s cervix begins to dilate too early in the pregnancy without perceived contractions or pain, it is called cervical insufficiency/incompetent cervix and commonly results in second-trimester pregnancy loss. An incompetent cervix can be due to congenital malformations or may develop from physical trauma like a D & C or difficult childbirth. 


The placenta is quite literally a baby’s lifeline to its mother. Babies receive oxygen and other nutrients from the placenta, and the placenta carries waste and carbon dioxide from the fetus to the mother’s circulatory system. If the placenta does not develop normally, the blood supply of the fetus can be restricted and result in a miscarriage. 

Placenta problems linked with second and third-trimester pregnancy complications and miscarriage include: 

  • Placental Abruption:  when the placenta separates from the uterine wall, this separation can be life-threatening for the fetus. Pregnant women may experience excessive blood loss. An abruption requires immediate delivery. 
  • Placenta Previa: When the placenta grows too close to the cervix, this often results in vaginal bleeding during pregnancy. Previa during labor can be fatal for a mother and her baby. 
  • Placenta Accreta/increta/percreta:  when the placenta connects too deeply into the uterus. This condition can cause a life-threatening during late pregnancy. These conditions might cause post-partum hemorrhage. Women often need a surgical procedure such as a hysterectomy to correct the problem and can cause death. 


Unhealthy lifestyle choices are common causes of miscarriage. The decision to smoke, drink alcohol, and use illegal drugs are all preventable causes of pregnancy loss. Excessive caffeine, exposure to toxic substances, and being over/underweight have also been associated with a higher risk of miscarriage. 

  • Smoking: Smoking and secondhand smoke may cause miscarriage. A systematic meta-analysis looking at 98 of the best controlled showed a large increase in the risk of miscarriage with cigarette smoking. Even second-hand smoke exposure leading to an overall increased risk of miscarriage by 11%.
  • Drinking Alcohol:  A large systematic meta-analysis of 24 of the best-conducted studies looking at over 200,000 women and the effects of alcohol consumption on miscarriage rates rose by 19% with women that had less than 5 drinks per week and 6% higher for those with more exposure.
  • Recreational & Other Illicit Drugs:  Research has linked miscarriages to a variety of drugs including marijuana and cocaine. Cocaine, can increase blood pressure and cause abruption, which can cause early preterm labor.

Getting The Help You Need

If you’ve had a miscarriage and would like to discuss your fertility options, CNY Fertility is ready to help no matter where you are in your journey.  Schedule your consultation today or give us a call at 844-315-2229. We’re here for you every step of the way!